Fuel-cell systems continue to offer much theoretical promise for the production of electricity, well over 150 years after the fuel cell was discovered and first demonstrated.
Today, hydrogen/oxygen fuel cells coupled with stored hydrogen are the farthest along the path toward commercialization of fuel cells. A major obstacle is hydrogen storage. For use in transportation applications, a gravimetric hydrogen density of at least 6 weight percent (wt %) for the entire hydrogen delivery system is generally desired.
High densities can be achieved using compressed hydrogen. However, the pressures required for reasonable volume densities are in excess of 350 atmospheres. Liquid hydrogen can also be used, but the energy required for liquefaction consumes a significant amount of energy, relative to the energy content of the stored hydrogen. In addition, maintaining liquid hydrogen for extended times without significant loss is difficult.
Hydrogen can be stored chemically in metal hydrides. Transition metals and alloys have been studied extensively for hydrogen-storage applications. The number of hydrogen atoms stored per metal atom generally does not exceed two, which largely precludes transition metals from achieving densities greater than about 5 wt % (typical values are about 1-3 wt %).
Lighter elements such as those in periods (rows) 2 and 3 in the periodic table can bond with hydrogen. One example is magnesium which can store about 7.5 wt % hydrogen as magnesium hydride, MgH2. However, magnesium hydride, and hydrides in general, are thermodynamically too stable, such that the hydride must be heated to excessively high temperatures to release the stored hydrogen. For example, MgH2 must be heated to temperatures of about 300° C. or greater to produce practical quantities (pressures) of hydrogen at equilibrium.
Some hydride systems have favorable thermodynamic (equilibrium) properties, but typically these systems have severe kinetics (rate) constraints. For example, sodium alanate, NaAlH4, stoichiometrically contains 5.6 wt % recoverable hydrogen that can be released by heating to temperatures of about 200° C., but the rate of H2 release is slow. Many complete metal-hydride systems involve multiple-phase solid-solid reactions which have very slow reaction rates. Complex metal hydrides also often suffer from instability toward water.
Regenerative fuel cell systems are capable of producing power and then electrolytically regenerating their reactants using electrochemical cells. Most regenerative fuel cells utilize the hydrogen/oxygen system. During the operation of such a fuel cell known in the art, hydrogen and oxygen combine to form water and generate electricity. Water can be electrolyzed back to hydrogen and oxygen, either within the same fuel cell or in a separate electrolyzer. While the system can function reversibly, hydrogen gas needs to be stored, usually in a tank. The weight of the tank significantly reduces the actual energy density achievable.
Direct-alcohol fuel cells use alcohol (e.g., methanol or ethanol) that is not reformed into hydrogen, but rather is fed directly into a fuel cell. Electrochemical oxidation of the alcohols, at the anode of the fuel cell provided by the prior art, leads to the formation of carbon dioxide. Practically speaking, such total oxidation to CO2 is an irreversible reaction. This irreversibility causes the continuous consumption of alcohol which therefore must be continuously provided to the fuel cell.
In view of these shortcomings associated with the prior art, what is needed is a fuel cell system that does not require costly means for hydrogen storage. What is further desired is a fuel cell system that reduces or eliminates carbon dioxide generation in the fuel cell, and is capable of being reversed to regenerate the reactant fuel.